According to this invention, olefin polymerization catalysts are prepared from supports comprising large pore volume xerogels or xerocogels. Some of the xerogels and xerocogels possess improved thermal stability. Chromium(II), chromium(III) or chromium(VI) compounds based on these supports are catalysts that are capable of producing polyethylene resins of melt index greater than about eight.
The improved large pore volume zirconia-silica catalyst supports are prepared by reacting a particular zirconium compound with a particular silicate to produce a hydrocogel, aging the hydrocogel, washing the hydrocogel, removing water from the resulting washed hydrocogel to produce a xerocogel either by azeotropic distillation or by leaching with a water miscible solvent, after which residual water is removed by chemically reacting this water with a ketal of the above formula RC(OR).sub.2 R, and then calcining the resulting xerocogel.
Another object of the invention is to provide a method of polymerizing olefins comprising contacting the olefins with a catalyst comprising the above support and a chromium compound associated with it under polymerizing conditions.
The following prior art is believed to be the most pertinent:
Aboutboul et al U.S. Pat. No. 3,978,002 discloses a chromium(VI)oxide olefin polymerization catalyst comprising a silica xerogel support obtained by dehydrating the corresponding hydrogel by extraction with acetone. A polymerization process using such a catalyst support for the production of ethylene polymers produces polyethylene resins having relatively low molecular weights and, therefore, high melt index in the range 3.2-4.2 (obtained without hydrogen modification).
Bachl et al U.S. Pat. No. 4,042,770 relates to a process for the manufacture of olefin polymers with the aid of a specially prepared silica-chromium(VI)oxide catalyst. The specially prepared silica hydrogel is dehydrated by extraction with an organic liquid selected from C.sub.1 -C.sub.4 alkanols and/or C.sub.3 -C.sub.5 alkanones (ketones) until the organic liquid absorbed no more water. The dehydrated gel is then freed of organic liquid by drying at 120.degree. C. to give the final xerogel. Doping with chromium(VI)oxide and air activation resulted in catalysts which produced ethylene polymers with very low melt index.
Short et al U.S. Pat. No. 4,081,407 describes a method for treating a hydrocogel consisting of coprecipitated silicatitania, 2.5 wt.% titanium (U.S. Pat. Nos. 3,950,316 and 3,862,104), with a saturated aliphatic alcohol selected from the group consisting of 1-pentanol, 3-methyl-1-butanol, 4-methyl-2-pentanol, 2,2-dimethyl-1-propanol, and 1-hexanol to remove water azeotropically or by extraction. The dried product retains a porous structure and is particularly suitable upon impregnation with a polymerization catalyst component, such as chromium(VI)oxide, for use in promoting the production of ethylene polymers of melt index in excess of about 6.6 (highest value disclosed is 7.5). These inventors discovered that the specific solvents disclosed removes water from the hydrocogels and affects the melt index of the polymer produced in polymerization reaction using catalysts supported on these strata.
Hwang et al U.S. Pat. No. 4,128,500 discloses chromium(III)acetylacetonate type catalysts. Dombro et al patent No. 4,246,137, the disclosure of which is hereby incorporated by reference, describe novel methods of preparing zirconia-silica xerocogels. Both are assigned to the assignee hereof.